This invention relates to a system for storing and utilizing thermal energy. More specifically, this invention relates to an integrated system for storing thermal energy, particularly low-grade thermal energy, and utilizing the thermal energy for space heating and cooling and for power generation.
Soaring demands for fossil energy coupled with dwindling supplies have stimulated a search for alternative, particularly renewable, sources of energy. It is well recognized that second law energy utilization efficiencies are very low (less than about 10%) for fissile or fossil fuels used directly or in the form of electricity in normal space heating and cooling applications. Since about 25% of the total energy consumption in the United States is for these purposes, an incentive exists for the utilization of lower grade heat sources better matched thermodynamically for space heating and cooling, since second law efficiencies would be much higher.
A number of sources of low-grade heat, i.e. temperatures up to about 150.degree. C., such as reject heat from central power stations, either fossil or nuclear fueled, geothermal energy and particularly solar energy, are in principle very well adapted to these applications. However, the successful use of energy from any of these sources, particularly for space heating and cooling, involves the storage of thermal energy. The intermittent availability of solar radiation in particular requires the storage of energy for use at times when solar radiation is not available. Traditional methods for storage of heat depend on heat capacity or phase change effects. Although water possesses a high specific heat, it has a low volumetric energy storage capacity and requires insulated holding tanks. Pebble beds are inexpensive but have the same drawbacks as water with a much lower heat capacity. Salt hydration-dehydration equilibria have a tendency toward supersaturation after many cycles, and stratification interferes with reversibility.
The use of hydrogen which can react exothermally with some intermetallic compounds to form decomposable hydrides has been suggested as a form of thermal energy storage. One such method and apparatus for storing thermal energy and for recovering the stored energy for space heating using metal hydrides is described in U.S. patent application Ser. No. 605,960, filed Aug. 19, 1975 and now abandoned.
Equally important with the ability to provide space heating in many parts of the country if the ability to also cool these same spaces. Particularly useful would be a single system which could provide total space air conditioning (both heating and cooling) using a source of low-grade thermal energy, since a large fraction of the power consumed today is for refrigeration or space cooling. One such system which can provide both heating and cooling in addition to energy conversion is disclosed in U.S. Pat. No. 3,943,719, issued Mar. 16, 1976. The described system produces high-pressure hydrogen by heating a metal hydride while retaining it at constant volume and generating power by expanding the compressed gas through a turbine or similar device. Thermal energy is recovered from the hot hydrogen gas before it is expanded, and refrigeration is supplied by using the expanded--and cold--hydrogen to absorb heat from a heat exchange fluid. However, the recovery of heat and refrigeration by this method is neither efficient nor effective for either space heating or cooling because it utilizes only the heat capacity of hydrogen gas.